LENGTH OF LIFE OF AN ANNUAL 301 



isms entails a course of life which embraces a senescent, or devel- 

 opmental period, and then a series of deteriorations resulting in 

 death, while the species is preserved by the activity of certain 

 rejuvenated portions of the body, which are specialized and cut 

 off during the period of activity. Many species of herbaceous 

 plants start from the seed, develop a shoot and form seeds, dying 

 in less than a hundred days from the time of germination. Others 

 develop two or more seasons before the capacity of forming seeds 

 is exhibited, and then make seeds one or many seasons, until 

 deterioration begins. 



Plants which live many seasons add numbers of mechanical 

 elements to their skeletons every year. The cells formed by the 

 generative layers, and which pass into permanent form, undergo 

 varying periods of senescence, and of endurance (356). 



The senescent changes in simple organisms, like Spirogyra and 

 other filamentous or unicellular organisms, are not well known, 

 and no systematic study of this phase of plant life has been made 

 in recent years. The cells soon reach an ultimate size in these 

 plants, and then carry on the normal functions for a time when 

 they begin to deteriorate. The duration of life of a higher plant 

 is so largely influenced by external conditions that it is difficult 

 to distinguish between phenomena of post-maturity, and those 

 due to lack of nutrition, harmful transpiration, parasites, ravages 

 of climate, etc. It may be easily seen upon theoretical grounds 

 however, that the mechanism of any plant is sufficient only to 

 serve its needs until a certain size is attained, and as a plant is 

 constantly increasing in size its age limit is also a limit of size. 

 Thus, for instance, a tree may grow only so long as its trunk 

 will support the constantly increasing crown. The difficulty of 

 supporting this crown will also be augmented in many instances 

 by air-currents. Then again, it is quite possible for a plant to 

 exhaust the food elements in the soil around the base of its stem, 

 and it must drive its rootlets a constantly increasing distance 

 through the substratum until the difficulty of transport of the soil 

 salts permits only an insufficient supply to reach the crown. 



